Rename system test directory with common files to _common

The old name "common" clashes with the convention of system test
directory naming. It appears as a system test directory, but it only
contains helper files.

To reduce confusion and to allow automatic detection of issues with
possibly missing test files, rename the helper directory to "_common".
The leading underscore indicates the directory is different and the its
name can no longer be confused with regular system test directories.

bin/tests/system/upforwd/tests.sh

@@ -20,7 +20,7 @@ set -e
 . ../conf.sh
 
 DIGOPTS="+tcp +noadd +nosea +nostat +noquest +nocomm +nocmd -p ${PORT}"
-RNDCCMD="$RNDC -p ${CONTROLPORT} -c ../common/rndc.conf"
+RNDCCMD="$RNDC -p ${CONTROLPORT} -c ../_common/rndc.conf"
 
 nextpart_thrice() {
 	nextpart ns1/named.run >/dev/null